Many hot air furnaces installed in homes or commercial structures in the past are only about 50-70% efficient. This is understandable since 15 or 20 years ago fuel costs were not a major concern and the furnaces sold reflected overall cost considerations of equipment and fuel. Today, the outlook has changed considerably--fuel costs have skyrocketed and fuel has become a depreciating commodity. With all fuels becoming limited, there is a pressing need to consume them at as high an efficiency rate as is possible given reasonable investment costs. To increase the efficiency of existing furnaces a retrofit system must be developed that provides low installed cost and low maintenance.
A hot air furnace is a structure commonly used today in homes. It transfers heat from the furnace to the room space by using circulating air as the heat transfer medium. The furnace can be fired by any fuel; however, the common fuels are either gas or oil. When the thermostat in the room space calls for heat, the burner in the furnace fires. When the heat exchanger reaches a minimum set temperature a thermal switch in the furnace turns on a blower which causes air to flow through the furnace heat exchanger into a hot air duct connected to the room space. Another duct returns air from the room space to the inlet of the blower. When the heat exchanger in the furnace reaches a maximum set temperature, the furnace thermal switch shuts off the burner. When the circulating air cools the heat exchanger back into its set operating range, the thermal switch refires the burner. When the room thermostat calls for a cessation of heating, the burner shuts off. When the furnace heat exchanger cools to the lower set temperature of the thermal switch, the blower shuts off. The present invention can be added to the hot air furnace without any change in the operating cycle described above.
Most existing hot air furnaces have low efficiency for conversion of the energy in the fuel to heat in the room space. Some local gas utilities find that conventional gas furnaces have a seasonal efficiency level in the range of 60-65%. New furnaces are available that have a level of 90-95%, but are considerably more expensive than conventional furnaces. The conventional furnaces were designed and sold when fuel was relatively low cost and when the cost of high-efficiency furnaces could not be justified. The conventional furnaces have limited heat transfer area in the furnace heat exchanger for cooling the combustion gases and they do not have a countercurrent flow arrangement between the flue gases and the circulating air. The present invention serves to correct both deficiencies by adding heat transfer area and by having the flue gases and the air in countercurrent heat exchange. This is done in a simple manner without the addition of blowers or controls so that the system can be easily applied to the very large number of existing furnaces.
The present invention can bring the furnace efficiency level from 60-65% to 80-85%. To raise the efficiency level higher would require condensing the moisture in the flue gas and would make the unit much more complex and costly. The estimated installed cost of the present invention is such that the fuel savings will pay for the unit in a very short period of time--even less than one year.
There have been many attempts to provide usable systems for flue gas heat recovery but as yet none that have both efficiency and are economical have been commercially acceptable. A system presently in limited use provides a simple heat exchanger in the flue pipe with a blower to pass air through the heat exchanger to recover some heat from the flue gas and to heat the area adjacent to the furnace. This system has low efficiency for heat recovery but has been used because of its simplicity and low costs. Other systems are described in U.S. Pat. Nos. 3,934,798; 4,122,999; 4,147,303; 4,308,990; 4,392,610; 4,408,716; and 4,418,866. In U.S. Pat. No. 3,934,798 a heat recovery system for hot air furnaces is described that has a heat exchanger installed in the flue gas line; however, it requires a separate duct from a room register and the system dumps the recovered heat into the cold air duct where it will cause a reduction in the heat transfer of the main furnace heat exchanger. The heat exchanger cannot tolerate any leakage since the air is at a slightly lower pressure than the flue gas. In U.S. Pat. No. 4,122,999 a forced air heating system is described that involves the installation and use of heat exchangers in the both the hot and cold air ducts of the furnace. A system such as this prior art structure requires integral heat exchangers and is not useful in retrofitting or converting existing low efficiency furnaces. U.S. Pat. Nos. 4,147,303 and 4,308,990 define a complex system that utilizes a heat-saving attachment that requires an added blower for air flow, requires ducts for heated and return air, and requires additional controls for the blower circuit. In U.S. Pat. Nos. 4,408,716 and 4,392,610, two added heat exchangers are required in their heat recovery systems since the recovered heat is transferred twice, i.e. flue gas to fluid and fluid to circulating air. In addition, the system described in these patents requires a fluid pump and requires controls for the hydraulic circuit. In U.S. Pat. No. 4,418,866 a heat recovery method is disclosed that takes air from the hot air duct of the furnace to exchange heat with flue gas at the elevated temperature of the hot air duct. Also, the method described in the patent requires a separate duct to move the heated air to the room space and requires a built in secondary heat exchange designed as an integral part of the furnace. Thus, none of the prior art patents disclose a heat recovery method, structure or system that is economical in installation and maintenance, substantially efficient and can be conveniently applied broadly to most existing furnaces.